1. Field of the Invention
The present invention relates to an optical disk apparatus for use with an optical disk to/from which data can be recorded and reproduced. More particularly, the present invention relates to an apparatus and a method for reproducing signals from an optical disk recording medium via an optical head, which are capable of reproducing address data with improved accuracy.
2. Description of the Related Art
The popularity of optical disk apparatuses such as CD (compact disk) players and DVD (digital video disk) players has been growing considerably in recent years. A large-capacity optical disk apparatus which allows a user to record data on an optical disk is expected to be commercially available in the near future.
FIG. 9 is a block diagram illustrating part of a conventional optical disk apparatus. An optical head 1001 irradiates a mark string 1000 formed on a recording surface of an optical disk recording medium with a laser beam and detects the reflected light therefrom as an electric signal HF. A comparator 1002 compares the amplitude of the signal HF with a threshold potential VTH and outputs a reproduced pulse signal DT. A charge pump 1003 charges a capacitor 1004 while the reproduced pulse signal DT is at a high level, and discharges the capacitor 1004 while the reproduced pulse signal DT is at a low level. The electric potential of the capacitor 1004 is provided to the comparator 1002 and used as the threshold potential VTH.
The operation of the conventional optical disk apparatus having such a structure will now be described. Binary (digital) data is recorded on the optical disk. More specifically, a mark string including a plurality of concave (or convex) marks is formed on the optical disk in accordance with the binary data to be recorded. The optical head 1001 reproduces a sinusoidal signal (not a pulse-like digital signal) from the mark string because of inter-symbol interference between adjacent marks. An appropriate threshold potential value VTH is provided to produce a pulse signal. Any amplitude in the sinusoidal signal greater than the threshold VTH is determined as a high level amplitude, whereas any amplitude less than the threshold VTH is determined as a low level amplitude.
As illustrated in the left-hand side of FIG. 10, when the threshold potential VTH is relatively low, the reproduced pulse signal DT becomes wide on the high level side and narrow on the low level side. As a result, the capacitor 1004 is charged more than it is discharged, thereby increasing the threshold potential VTH. Thus, the threshold potential VTH is controlled so that the average amount of current charged into the capacitor 1004 substantially equals the average amount of current discharged therefrom. In other words, the threshold potential VTH is controlled so that the average length of the "H" period of the reproduced pulse signal DT (a period during which the signal DT is at the high level) and the average length of the "L" period thereof (a period during which the signal DT is at the low level) are equal or at least closer to each other. The ratio between the "H" period and the "L" period is referred to as "the duty ratio" of the signal DT.
Such an optical disk apparatus may be advantageously used with recording media such as CDs and DVDs, where data is recorded based on the PWM (pulse width modulation) method. The PWM method is a recording method suitable in high density recording applications, where the length of a recording mark varies in accordance with the data to be recorded. When reproducing binary data based on the PWM method, however, even a slight shift in the threshold value VTH may cause an error in the pulse length of the reproduced pulse signal DT, thereby resulting in a reproduction error.
In view of this, feedback control may be constantly performed for the threshold value VTH so that the duty ratio of the reproduced pulse signal is substantially constant as described above, thereby reproducing data without an error (see Japanese Laid-open Publication No. 63-201957).
However, such a method assumes that data is recorded continuously without interruption. When PWM data segments (data segments which are recorded based on the PWM method) exist at intervals on the optical disk, the threshold value VTH follows (varies in accordance with) noise when an optical head scans over an area with no recorded data.
As described above, the conventional method is used for reproducing data from read-only media such as CDs and DVDs, where data is continuously recorded based on the PWM method across the entire surface of the disk. However, an optical disk and an optical disk apparatus which allow a user to record data on the disk are expected to be commercially available in the near future. While several different recording formats have been proposed, such a recordable optical disk typically includes address areas and data areas which are arranged alternately at predetermined intervals. In the data area, a film (e.g., a phase change material film or a magneto-optical recording film) to which data can be recorded by laser heat is provided. In the address area, address data has been recorded as concave or convex marks. Since a recordable optical disk is also desired to have a higher recording density, the PWM method should be applied to the address area as well as to the data area. However, such a "recordable" medium may have a data area with no recorded data. In such a case, only the address area has PWM concave or convex marks (marks recorded based on the PWM method). When this recordable optical disk is reproduced by the conventional method, the unrecorded area is reproduced as a long low-level signal. In response, the feedback control system tries to decrease the threshold value VTH as low as possible. Thus, the threshold value VTH follows the noise, and the optical disk apparatus generates undesired signals by digitizing the noise. As a result, it is not possible to identify the correct address.